Polishing liquid, method for manufacturing glass substrate, and method for manufacturing magnetic disk

ABSTRACT

Letting a particle diameter be Dx (μm) when a cumulative particle volume cumulated from the small particle diameter side reaches x (%) of the total particle volume in a particle size distribution obtained regarding cerium oxide included in a polishing liquid using a laser diffraction/scattering method, D5 is 1 μm or less, and a difference between D95 and D5 is 3 μm or more.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. patentapplication Ser. No. 16/641,185, filed on Feb. 21, 2020, which is a U.S.National stage application of International Patent Application No.PCT/JP2018/040610, filed on Oct. 31, 2018, which claims priority toJapanese Patent Application No. 2017-210817, filed in Japan on Oct. 31,2017. The entire contents of U.S. patent application Ser. No. 16/641,185and Japanese Patent Application No. 2017-210817 are hereby incorporatedherein by reference.

BACKGROUND Field of the Invention

The present invention relates to a polishing liquid that contains ceriumoxide as loose abrasive particles and is used in polishing processingfor polishing a surface of a glass substrate, a method for manufacturinga glass substrate with use of this polishing liquid, and a method formanufacturing a magnetic disk.

Background Information

Nowadays, in order to record data, hard disk drives (HDDs) areincorporated in personal computers, DVD (Digital Versatile Disc)recording apparatuses, and the like.

A magnetic disk obtained by providing a magnetic layer on a substrate isused in a hard disk drive, and magnetic recording information isrecorded in or read from the magnetic layer with a magnetic head that isslightly away from the surface of the magnetic disk. In order toincrease the storage capacity in recent hard disk apparatuses, attemptshave been made to increase the magnetic recording density. In order tomake it possible to increase the magnetic recording density, surfaceunevenness of main surfaces of a glass substrate to be used as amagnetic-disk substrate needs to be reduced as much as possible. Also,accompanying this, there is demand for also making surface unevenness ofinner and outer circumferential edge surfaces of a glass substrate assmall as possible so that no microparticles adhere to the slight gapsformed between protrusions and recesses of the edge surfaces scatter andare transferred to the main surfaces of the glass substrate.

Incidentally, if processing for polishing a glass substrate isperformed, there are cases where cerium oxide is used as loose abrasiveparticles. Cerium oxide is used as loose abrasive particles in oneprocess for polishing a main surface of a glass substrate, and inprocessing for polishing inner and outer circumferential edge surfacesof a glass substrate, for example.

When a magnetic-disk glass substrate is manufactured, for example, innerand outer circumferential edge surfaces of a glass material arechamfered, and then, inner and outer circumferential edge surface mirrorprocessing is performed through brush polishing with use of a brush anda polishing liquid that contains cerium oxide as loose abrasiveparticles (e.g., JP 2012-20377A).

SUMMARY

If such cerium oxide is used as loose abrasive particles in a polishingliquid, the polishing rate may decrease even if polishing processing isperformed under the same polishing processing conditions. In this case,polishing processing conditions need to be changed according to changesin the polishing rate. Also, there is an issue regarding a productionefficiency, such as a need to increase the frequency of replacing apolishing liquid that contains cerium oxide with a new polishing liquid.Also, because cerium oxide is relatively expensive, increasing thefrequency of replacement will increase production costs.

In view of this, an object of the present invention is to provide apolishing liquid that can be repeatedly used in polishing processing fora long time and in which even if a polishing liquid containing ceriumoxide as loose abrasive particles is repeatedly used in polishingprocessing for polishing a glass substrate with use of the polishingliquid, a change in the polishing rate is slight, and also to provide amethod for manufacturing a glass substrate with use of this polishingliquid, and a method for manufacturing a magnetic disk.

One aspect of the present invention is a polishing liquid that containscerium oxide as loose abrasive particles and is to be used in polishingprocessing for polishing a surface of a glass substrate.

Letting a particle diameter be Dx [μm] when a cumulative particle volumecumulated from the small particle diameter side reaches x [%] of a totalparticle volume in a particle size distribution obtained regarding thecerium oxide using a laser diffraction/scattering method,

D5 is 1 μm or less, and a difference between D95 and D5 is 3 μm or more.

It is preferable that the difference between the D95 and the D5 is 4 μmor more.

It is preferable that the D95 is 7 μm or less.

It is preferable to use the polishing liquid in polishing processing forpolishing an edge surface of a glass substrate with use of a brush.

Another aspect of the present invention is a method for manufacturing aglass substrate, the method including processing for polishing a surfaceof a substrate. The polishing in this manufacturing method is processingfor polishing the glass substrate with use of the polishing liquid.

It is preferable that the glass substrate is a magnetic-disk glasssubstrate.

It is preferable that the magnetic-disk glass substrate has an outercircumferential diameter of 94 mm or more.

Yet another aspect of the present invention is a method formanufacturing a magnetic disk, the method including processing forforming at least a magnetic layer on a main surface of the magnetic-diskglass substrate obtained using the glass substrate manufacturing method.

According to the above-described polishing liquid, even if the polishingliquid containing cerium oxide as loose abrasive particles is repeatedlyused in polishing processing for polishing a glass substrate with use ofthe polishing liquid, a change in the polishing rate is slight, and itis possible to repeatedly use this polishing liquid in polishingprocessing for a long time. Thus, even if the above-described polishingliquid is repeatedly used in polishing processing, the frequency ofchanging the polishing processing conditions is low, and the samepolishing liquid can be used in polishing processing for a long time.According to the above-described glass substrate manufacturing methodand magnetic disk manufacturing method, polishing processing can bestably performed with use of the above-described polishing liquid, andthus it is possible to stably manufacture glass substrates and magneticdisks.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing one example of a particle sizedistribution of loose abrasive particles in one embodiment.

FIG. 2 is a schematic diagram showing one example of a particle sizedistribution of conventional loose abrasive particles.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Hereinafter, a polishing liquid, a method for manufacturing a glasssubstrate, and a method for manufacturing a magnetic disk according toan embodiment of the present invention will be described in detail. Notethat the method for manufacturing a glass substrate according to thisembodiment is suitable for manufacturing a magnetic-disk glass substratehaving a nominal size of 2.5 to 3.5 inches (e.g., having a diameter of53 to 98 mm), and having a substrate thickness of 0.3 to 2.0 mm, forexample.

The inventor of the present invention found that, when the inventorstudied polishing processing in which a polishing liquid containingcerium oxide as loose abrasive particles was used, cerium oxideparticles included relatively fragile particles, and abrasive particleswere broken by polishing processing and a particle size distribution waslikely to change. If a polishing liquid is repeatedly used in polishingprocessing, a large particle is divided into two, for example, and theratio of relatively small particles rapidly increases. Note that theinventor also found that, usually, the frequency of large particles wasnot completely zero because some particles were very hard andunbreakable. It is inferred that, even if a large particle breaks, asmall particle having a particle diameter of 1 μm or less is likely toseparate from the large particle to be peeled off, instead of the largeparticle being divided at the center thereof into two with thesubstantially equivalent size, and a large particle is yet maintained.That is, a particle size distribution showing the frequency with respectto the particle diameter of cerium oxide changes to a broad particlesize distribution spreading toward the small particle diameter sidebecause as the distribution spreads toward the small particle diameterside, the frequency rapidly increases, and the frequency on the largeparticle diameter side decreases. Thus, the inventor found that it wasdifficult to maintain, for a long time, a polishing rate at thebeginning of use using a polishing liquid with a uniform particlediameter, that is, a polishing liquid in which the frequency of particlediameters is concentrated in a narrow range of the particle diameter.

The inventor examined, based on such findings, a change in the polishingrate of a polishing liquid at the beginning of use by changing afrequency distribution (a particle size distribution) of particlediameters of cerium oxide in various ways. As a result, the inventorfound that as a result of using, in a polishing liquid, cerium oxidewith a predetermined distribution that somewhat includes a smallparticle diameter region, the predetermined distribution being aparticle size distribution of loose abrasive particles that is broaderthan a conventional particle size distribution, even if polishingprocessing is repeated, the influence of a change in the particle sizedistribution can be reduced, and a change in the polishing rate issmall, and such a polishing liquid can be repeatedly used in polishingprocessing for a long time. The inventor found a polishing liquid havingthe following aspects under such a background.

Although a glass blank that has undergone shape processing performedwhen a magnetic-disk glass substrate is manufactured will be describedmainly as a substrate to be subjected to polishing processing with useof a polishing liquid in the following description, the substrate to besubjected to polishing processing is not limited to a glass blank for amagnetic-disk glass substrate, and there is no particular limitation ona substrate to be subjected to polishing processing as long as asubstrate is a glass substrate on which polishing processing isperformed using cerium oxide as loose abrasive particles. Examplesthereof include main surfaces of a glass blank on which edge surfacepolishing processing has been performed, that is, main surfaces of anintermediate glass substrate, and main surfaces of a glass substrate fora mask blank, a glass substrate for liquid crystal, a glass substratefor cover glass, and glass for an optical component such as a lens. Aglass blank or an intermediate glass substrate will be collectivelyreferred to as a “glass substrate” hereinafter, but will also bereferred to as a “glass blank” or an “intermediate glass substrate” asneeded.

In a process for manufacturing a magnetic-disk glass substrate, surfaceunevenness is reduced through processing for polishing main surfaces orinner and outer circumferential edge surfaces of a glass substrate. Insuch a case, cerium oxide is used in a polishing liquid as looseabrasive particles. The particle diameter of loose abrasive particles issomewhat limited according to a polishing rate and the required qualityof a polished surface, and an average particle diameter thereof is set.However, the polishing rate changes as a result of such loose abrasiveparticles being used in polishing processing for a long time. Thepolishing rate is likely to change depending on the particle diameter ofloose abrasive particles, and in particular, the polishing rate islikely to significantly decrease because cerium oxide used as looseabrasive particles include particles that are likely to break.

As described above, a decrease in the polishing rate is caused by thefact that cerium oxide particles with a large particle diameter thatmake large contributions to a polishing rate are broken throughpolishing processing to become small particles and the particle diameterfrequency distribution changes. Thus, if cerium oxide that has aparticle diameter frequency distribution that does not significantlychange even if cerium oxide particles having a large particle diameterare broken by performing polishing processing is used and polishingprocessing conditions are set, even if polishing processing is repeatedwith use of this polishing liquid, a change in the particle sizedistribution can be reduced, and a change in the polishing rate can bereduced.

A polishing liquid used in polishing processing of one embodiment is asfollows.

Letting a particle diameter (the diameter of abrasive particles) be Dx[μm] when a cumulative particle volume cumulated from the small particlediameter (microparticles) side reaches x [%] of a total particle volumein a particle size distribution obtained regarding cerium oxide includedin the polishing liquid using a laser diffraction/scattering method, D5is 1 μm or less, and a difference between D95 and D5 is 3 μm or more.

In other words, when a particle diameter cumulative frequency obtainedby cumulating the frequency from the minimum particle diameter of ceriumoxide to a predetermined particle diameter in a particle sizedistribution (a particle size distribution based on volume) thatindicates the frequency [%] with respect to the particle diameter ofcerium oxide included in the polishing liquid is x [%] (x indicates anumber of 0 to 100 inclusive) and a predetermined particle diameter isDx [μm], D5 is 1 μm or less, and a difference between D95 and D5 is 3 μmor more.

Even if such a polishing liquid is repeatedly used in polishingprocessing for a long time, a decrease in the polishing rate is small,the frequency of changing the polishing processing conditions can bereduced, and the same polishing liquid can be repeatedly used for a longtime. Thus, polishing processing can be stably performed with use of theabove-described polishing liquid, and thus it is possible to stablymanufacture glass substrates and magnetic disks. Also, a decrease in thequality of a polished surface is small even if such a polishing liquidis repeatedly used in polishing processing for a long time, and thefrequency of changing the polishing processing conditions can bereduced. Thus, the same polishing liquid can be repeatedly used for along time, and thus it is possible to stably perform polishingprocessing with use of the above-described polishing liquid, and tostably manufacture glass substrates and magnetic disks. The surfacequality refers to the degree of unevenness of a polished surface or thenumber of defects such as pits and scratches when polishing is performedat the same polishing machining allowance amount. A low surface qualityrefers that unevenness of a polished surface is high, or the number ofdefects such as pits and scratches is high.

If a magnetic-disk glass substrate has an outer circumferential diameterof 94 mm or more (a nominal size of 3.5 inches or more), an edge surfacepolishing amount increases due to the outer circumferential surfacebeing longer, compared to a conventional nominal size of 2.5 inches orless (e.g., the outer circumferential diameter is 53 to 68 mm), and thusthe polishing rate decreases, and the frequency of replacement of thepolishing liquid also increases. Thus, the amount of the consumedpolishing liquid increases and productivity significantly decreases.However, use of the polishing liquid of this embodiment makes itpossible to minimize a decrease in the productivity of glass substrates.

Also, a magnetic-disk glass substrate more preferably has a substratethickness of 0.3 to 0.64 mm. If edge surface polishing processing isperformed with use of a brush, for example, the thinner the substrateis, the more difficult to carry out edge surface polishing, and it isimportant to repeatedly supply a polishing liquid. Thus, the polishingliquid of this embodiment with a particle size distribution of looseabrasive particles being restricted taking a change in the polishingrate into consideration has a large effect on suppressing a decrease inproductivity in the case where a substrate has a thickness of 0.3 to0.64 mm.

Hereinafter, a glass substrate on which polishing processing isperformed with use of the above-described polishing liquid will bedescribed. In the following description, an example in which inner andouter circumferential edge surfaces of a glass substrate are polishedmainly as surfaces of a glass substrate to be subjected to polishingprocessing will be described. However, surfaces of a glass substrate tobe subjected to polishing processing may be main surfaces (surfaces withthe largest area) of the glass substrate.

Magnetic-Disk Glass Substrate

A magnetic-disk glass substrate has a circular plate shape, and acentral hole having a circular shape concentric with the outercircumference is hollowed out so that the magnetic-disk glass substratehas a ring shape. A magnetic disk is formed by forming magnetic layers(recording regions) in annular regions of both surfaces of themagnetic-disk glass substrate.

The glass blank for a magnetic disk (hereinafter simply referred to as“glass blank”) is a circular glass plate produced through press molding,for example, and is in a form in which the central hole has not yet beenhollowed out. Also, a glass blank can be taken out from a glass plateobtained using a float method or a downdraw method, and in this case, aglass blank may be obtained by forming a central hole when taking out aglass blank. Aluminosilicate glass, soda-lime glass, borosilicate glass,and the like can be used as a material of the glass blank. Inparticular, aluminosilicate glass can be suitably used in light of thefact that chemical strengthening can be carried out, and a magnetic-diskglass substrate with better flatness of main surfaces and substratehardness can be produced.

Method for Manufacturing Magnetic-Disk Glass Substrate

Next, a method for manufacturing a magnetic-disk glass substrate will bedescribed. First, a glass blank that is a raw material for aplate-shaped magnetic-disk glass substrate having a pair of mainsurfaces is produced through press molding (press-molding processing).Then, a circular hole is formed in a central portion of the producedglass blank, and the resulting glass blank has a ring shape (annularshape) (circular hole formation processing), and then, shaping isperformed (shape processing). Accordingly, a glass substrate (a glassblank) having chamfered surfaces is produced. Then, polishing isperformed on inner and outer circumferential edge surfaces of the glasssubstrate (the glass blank) that has been subjected to shaping (edgesurface polishing processing). Grinding with fixed abrasive particles isperformed on the main surfaces of the glass substrate on which edgesurface polishing has been performed, that is, the main surfaces of theintermediate glass substrate (grinding processing). Then, firstpolishing is performed on the main surfaces of the glass substrate (theintermediate glass substrate) (first polishing processing). Then,chemical strengthening is performed on the glass substrate (theintermediate glass substrate) (chemical strengthening processing). Notethat chemical strengthening processing need not be performed. Then,second polishing is performed on the chemically strengthened glasssubstrate (the intermediate glass substrate) (second polishingprocessing). A glass substrate that is a source of a magnetic disk (aglass substrate as a final product) can be obtained through theabove-described processing. Hereinafter, each process will be describedin detail.

(a) Press-Molding Processing

A glass blank is molded by cutting a leading end of a molten glass flowwith use of a cutter, and pressing the mass of the cut molten glassbetween press molding surfaces of a pair of molds. After pressing isperformed for a predetermined time, the molds are opened to remove theglass blank.

(b) Circular Hole Formation Processing

A disk-shaped glass substrate (a glass blank) having a circular hole canbe obtained by forming the circular hole in the glass blank with use ofa drill or the like. A circular hole is formed by forming a scribe lineon a main surface of the glass blank in a circular shape with use of ascriber, applying heat to extend a crack along the scribe line, andseparating a circular inner portion, for example.

(c) Shape Processing

In the shape processing, chamfering processing is performed on an endportion of the glass substrate (the glass blank) that has undergonecircular hole formation processing. Chamfering is performed with use ofa formed grindstone, for example.

(d) Edge Surface Polishing Processing

In the edge surface polishing processing, finishing is performed on theinner and outer circumferential side edge surfaces of the glasssubstrate (the glass blank) through brush polishing. At this time, apolishing liquid containing cerium oxide microparticles having apredetermined particle size distribution as loose abrasive particles,that is, as polishing abrasive particles, is used. The cerium oxideparticle size distribution will be described later. It is preferablethat the edge surfaces of the glass substrate (the glass blank) becomemirror surfaces through this polishing processing. The glass blank thatwas subjected to edge surface polishing processing is an intermediateglass substrate.

(e) Grinding Processing

In the grinding processing, grinding processing is performed on the mainsurfaces of the glass substrate that was subjected to edge surfacepolishing processing, that is, the main surfaces of the intermediateglass substrate, using a double-side grinding apparatus provided with aplanetary gear mechanism. Specifically, two main surfaces of the glasssubstrate (the intermediate glass substrate) are ground while the outercircumferential edge surface of the glass substrate (the intermediateglass substrate) is held in a holding hole provided in a holding memberof the double-side grinding apparatus. The double-side grindingapparatus has a pair of upper and lower surface plates (an upper surfaceplate and a lower surface plate), and the glass substrate is heldbetween the upper surface plate and the lower surface plate. Then, thetwo main surfaces of the glass substrate can be ground by moving theglass substrate (the intermediate glass substrate) and the surfaceplates relative to each other by moving one or both of the upper surfaceplate and the lower surface plate.

(f) First Polishing Processing

First polishing is performed in order to remove blemishes anddeformation and adjust minute unevenness (microwaviness, roughness)remaining on the main surfaces in the case where grinding with fixedabrasive particles is performed, for example. Specifically, the two mainsurfaces of the glass substrate (the intermediate glass substrate) arepolished while the outer circumferential edge surface of the glasssubstrate (the intermediate glass substrate) is held in a holding holeprovided in a carrier for polishing of a double-side polishingapparatus. It is preferable that the main surfaces of the glasssubstrate (the intermediate glass substrate) are mirror surfaces afterthe first polishing processing is performed.

In the first polishing processing, the glass substrate (the intermediateglass substrate) is polished using a double-side polishing apparatushaving a configuration similar to that of the double-side grindingapparatus that is used in grinding processing with fixed abrasiveparticles, while a polishing liquid is provided. In the first polishingprocessing, unlike grinding with fixed abrasive particles, a polishingliquid containing loose abrasive particles is used, instead of fixedabrasive particles. Cerium oxide abrasive particles, cerium hydroxideabrasive particles, zirconia abrasive particles, zirconium silicateabrasive particles, or the like are used as loose abrasive particlesused in first polishing, for example. In particular, in order to achievea good polishing rate, it is preferable to use cerium oxide or zirconia,and it is most preferable to use cerium oxide as loose abrasiveparticles. If cerium oxide is used as loose abrasive particles, as willbe described later, it is preferable to use cerium oxide having apredetermined broad particle size distribution that spreads toward thesmall particle diameter side such that the characteristics of theparticle size distribution are unlikely to change even if a particlewith a large particle diameter changes to a particle with a smallparticle diameter, in light of the fact that a stable polishing rate andpolishing quality can be ensured.

(g) Chemical Strengthening Processing

In the chemical strengthening processing, the glass substrate (theintermediate glass substrate) is chemically strengthened by immersingthe glass substrate (the intermediate glass substrate) in a chemicalstrengthening liquid. A melt in which potassium nitrate and sodiumnitrate are mixed, for example, can be used as the chemicalstrengthening liquid.

(h) Second Polishing (Final Polishing) Processing

The second polishing processing is performed in order to mirror-polishthe main surfaces of the glass substrate (the intermediate glasssubstrate). A double-side polishing apparatus having a configurationsimilar to that of the double-side polishing apparatus that is used infirst polishing is used in second polishing as well. Specifically, twomain surfaces of the glass substrate (the intermediate glass substrate)are polished while the outer circumferential edge surface of the glasssubstrate (the intermediate glass substrate) is held in a holding holeprovided in a carrier for polishing of the double-side polishingapparatus. The machining allowance for second polishing is about 1 to 10μm, for example. In the second polishing processing, the type and theparticle size of loose abrasive particles are changed, and the hardnessof a resin polisher is changed, with respect to the first polishingprocessing. Specifically, a polishing liquid containing silicon dioxidemicroparticles having a particle diameter of about 5 to 100 nm as looseabrasive particles is supplied between the polishing pads of thedouble-side polishing apparatus and the main surfaces of the glasssubstrate (the intermediate glass substrate), and the main surfaces ofthe glass substrate (the intermediate glass substrate) are polished. Itis preferable that silicon dioxide microparticles are colloidal silica.The polished glass substrate (the intermediate glass substrate) iscleaned using a neutral detergent, pure water, isopropyl alcohol, or thelike, and thus a magnetic-disk glass substrate that is a source of amagnetic disk (a glass substrate as a final product) can be obtained.

Carrying out the second polishing processing makes it possible to setthe roughness (Ra) of the main surfaces thereof to 0.3 nm or less, andmore preferably 0.1 nm or less. Also, microwaviness of the main surfacescan be set to 0.1 nm or less. In this manner, the glass substrate thatwas subjected to second polishing (a glass substrate as a final product)is cleaned and dried as appropriate to be a magnetic-disk glasssubstrate.

Then, a magnetic disk can be obtained by forming at least a magneticlayer on a surface of the magnetic-disk glass substrate.

(Loose Abrasive Particles)

Next, the particle size distribution of cerium oxide included as looseabrasive particles in the polishing liquid used in the above-describededge surface polishing processing of one embodiment will be described.The concentration of cerium oxide in the polishing liquid is 1 to 30mass %, for example. The particle size distribution of loose abrasiveparticles can be obtained using a particle size distribution measurementapparatus with use of a laser diffraction/scattering method. In thecerium oxide particle size distribution, usually, the frequency isdeviated toward the small particle diameter side, and thus, with regardto the horizontal axis (the particle diameter) when obtaining theparticle size distribution, it is sufficient to convert the particlediameter into the logarithm, and plot the logarithm of the particlediameter on the horizontal axis. It is sufficient to then adjust aparticle diameter range for classifying particle diameter ranges withthe number of intervals between D5 to D95 being about 10 to 20.

According to one embodiment, the polishing liquid used in polishingprocessing further contains a predetermined dispersion medium (e.g.,water) and loose abrasive particles dispersed in the dispersion medium,and further contains a dispersing agent for dispersing loose abrasiveparticles in the dispersion medium as needed.

D5 of cerium oxide included as loose abrasive particles in the polishingliquid of one embodiment is 1 μm or less, and the difference between D95and D5 is 3 μm or more. Here, Dx [μm] refers to a particle diameter (thediameter of particles) when a cumulative particle volume cumulated fromthe small particle diameter side reaches x [%] of a total particlevolume in a particle size distribution obtained using a laserdiffraction/scattering method. In other words, Dx [μm] refers to theabove-described predetermined particle diameter when the cumulativefrequency obtained by cumulating the frequency from the minimum particlediameter of cerium oxide to the predetermined particle diameter thereofis x [%]. Thus, D100 refers to the maximum particle diameter in looseabrasive particles. Thus, D5 indicates the particle diameter that isrelatively close to the minimum particle diameter, and D95 indicates theparticle diameter that is close to the maximum particle diameter. Thus,D95 is larger than D5. When the difference between these two particlediameters is 3 μm or more, it means that the cerium oxide particle sizedistribution is broader than a conventional particle size distribution.Setting D5 to 1 μm or less makes it possible to ensure the quality ofthe polished inner and outer circumferential edge surfaces of a glasssubstrate (an intermediate glass substrate) in predetermined ranges.

Note that if D5 is less than 0.5 μm, there are cases where a polishingrate decreases because an excessively large number of abrasive particleswith small diameters are present. Thus, D5 is more preferably 0.5 μm to1 μm inclusive.

Also, it is preferable that the number of peaks of the relativefrequency in the cerium oxide particle size distribution to be measuredis two or less. If there are three or more peaks, there is a possibilitythat it will be difficult to manage a particle size distribution. Also,if there are two peaks, it is preferable that the peak on the largeparticle diameter side is higher than the other in light of the factthat a high polishing rate is obtained at the beginning of continuouspolishing processing. If there are two peaks, the two peaks arepreferably present in a particle diameter range of D10 to D85. Although,if the two peaks are present in this range, the polishing rate andpolishing quality exhibit properties that are substantially equivalentto those when there is one peak, if a peak departs from this range,there are cases where the number of surface defects increases.

Also, the upper limit of the difference between D95 and D5 is preferably10 μm, and more preferably 7 μm. If the difference therebetween isexcessively large, there is a risk that the number of surface defectssuch as pits and scratches will increase.

FIG. 1 is a schematic diagram showing one example of a particle sizedistribution of loose abrasive particles in one embodiment, and FIG. 2is a schematic diagram showing one example of a particle sizedistribution of conventional loose abrasive particles. The solid lineindicates a particle size distribution of loose abrasive particlesbefore polishing processing, and the broken line indicates a particlesize distribution obtained after polishing processing is continuouslyperformed on ten batches while circulating a polishing liquid, forexample. The polishing liquid is not partially replenished or replacedduring this processing.

D5 and D95 are shown in FIGS. 1 and 2. The particle size distributionindicated by the solid line in FIG. 1 is a broad distribution in which aparticle diameter range is wider and the maximum value of the relativefrequency (the value of the frequency when the frequency is at themaximum) is lower than those of the particle size distribution indicatedby the solid line in FIG. 2, the distribution spreading on the smallparticle diameter side, compared to the particle size distribution shownin FIG. 2. If a polishing liquid containing loose abrasive particleshaving such a particle size distribution is continuously used inpolishing processing, a portion of a large particle breaks through thepolishing processing to split into two or more small particles, thusshowing a particle size distribution as the distributions indicated bythe broken lines in FIGS. 1 and 2. Specifically, the overall particlesize distribution shifts in a direction toward the small particlediameter side from the particle size distribution indicated by the solidline. Although, as shown in FIGS. 1 and 2, the frequency of the particlediameter near the maximum particle diameter is reduced, as describedabove, particles that are unlikely to break are included therein, andthus the maximum particle diameter hardly decreases.

At this time, a range where the solid line and the broken line overlapeach other is wide in the particle size distribution shown in FIG. 1,and a range where the solid line and the broken line overlap each otheris narrow in the particle size distribution shown in FIG. 2. Thus, ifthe polishing liquid is used in polishing processing for a long time, achange in the polishing rate of the polishing liquid containing looseabrasive particles having the particle size distribution shown in FIG. 1is smaller than that of the polishing liquid containing loose abrasiveparticles having the particle size distribution shown in FIG. 2.

In this case, a particle diameter Dpeak (mode particle diameter) atwhich the frequency is at the maximum in the above-described particlesize distribution is preferably 1 μm or more. If the particle diameterDpeak is less than 1 μm, there are cases where the polishing ratedecreases from the beginning of use of the polishing liquid and thepolishing processing time extends. Also, if the particle diameter Dpeakis excessively large, there are cases where surface defects on a surfaceto be polished increase, and thus the Dpeak is preferably 5 μm or less,and more preferably 3 μm or less. Also, D50 is preferably 0.8 to 2.4 μmin the light of inhibiting an overall change in the particle sizedistribution. Also, D100 (the maximum particle diameter) is preferably15 μm or less in the light of reducing surface defects on a surface tobe polished. Note that D100 is preferably 3 μm or more in the light ofobtaining a broad particle size distribution. It is preferable thatDpeak-D5 is smaller than D95-Dpeak. Also, it is preferable that D50-D5is smaller than D95-D50. The reasons for this are that, because largeabrasive particles are gradually broken into small abrasive particlesthrough polishing processing, when (Dpeak-D5)<(D95-Dpeak) holds true, achange in the particle size distribution thereof is likely to decreaseoverall. It is preferable that D50-D5 is smaller than D95-D50 due to thesame reasons.

According to one embodiment, it is preferable that a difference betweenD95 and D5 is 4 μm or more. This makes it possible to further suppress adecrease in the polishing rate even if the polishing liquid is used fora long time.

According to one embodiment, it is preferable that D95 is 7 μm or less.This makes it possible to maintain the quality of the polished surfacein an allowable range.

In one embodiment, when the polishing liquid is used in polishingprocessing for polishing edge surfaces of a glass substrate (a glassblank) with use of a brush, the polishing rate of the polishing liquidand a decrease in the quality of the polished surface can be furthereffectively inhibited. In the polishing of edge surfaces of a glasssubstrate (a glass blank) with use of a brush (brush polishing), apredetermined number of glass substrates (glass blanks) are stacked toproduce a stacked body such that main surfaces of the glass substratesface each other, and side surfaces of the stacked body (outercircumferential edge surfaces or inner circumferential edge surfaces ofthe glass substrates (the glass blanks)) are collectively polished withuse of the brush, for example. In the case of brush polishing, becauseleading ends of the brush are obliquely pressed against the surfaces tobe polished in a bent or bundled state, gaps between the surfaces to bepolished and the brush vary from a very small size to a relatively largesize and are not uniform. As a result, abrasive particles with variousparticle diameters fit in appropriate gaps and exert polishing effects.That is, abrasive particles with various particle diameters are pressedagainst the surfaces thereof by the brush at the same time, and thus thewidth of the particle size distribution (the difference between D95 andD5) has a significant influence. From this point of view, a largereffect can be exhibited by applying the polishing liquid of thisembodiment to brush polishing.

Examples of the material of the bristles constituting the brush includepolyamide synthetic fibers, PBT (polybutylene terephthalate), and PP(polypropylene). The wire diameter of the bristle material is about 0.05to 1 mm, for example.

Note that even in a case where soft polishing pads made of polyurethanefoam or the like are used in order to polish surfaces of glasssubstrates (intermediate glass substrates), the pads may be in the samestate as in brush polishing (the state in which abrasive particles withvarious particle diameters are pressed against the surfaces thereof bythe polishing pads at the same time). Thus, even if soft polishing padsare used, the polishing liquid of this embodiment can be effectivelyused. In particular, swede polishing pads made of polyurethane foam aresuitable because surface defects can be easily reduced due to thesurfaces thereof being relatively soft.

Thus, as a result of performing polishing processing on glass substrates(glass blanks or intermediate glass substrates) with use of such apolishing liquid, the intermediate glass substrates, or glass substratesas final products, can be stably manufactured without frequentlychanging polishing processing conditions. Thus, it is possible to stablyuse the method for manufacturing a magnetic disk with which at least amagnetic layer is formed on main surfaces of the manufactured glasssubstrate.

(Experimental Examples)

In order to check the effects of the polishing liquid according to anembodiment, polishing liquids (with a concentration of cerium oxidebeing 10 mass %) obtained by changing a cerium oxide particle sizedistribution in various ways were produced, and outer circumferentialedge surfaces of glass substrates (glass blanks) were subjected to edgesurface polishing. This edge surface polishing is brush polishing withuse of a brush. In this brush polishing, a polishing liquid was notreplaced, and 20 batches were continuously processed while the polishingliquid was circulated, with polishing processing conditions (the numberof stacked glass substrates (glass blanks), the polishing time, theconcentration of a polishing liquid, and the pressing force of a brush)for one batch kept constant. The polishing rate for glass substrates(intermediate glass substrates) in the first batch and the polishingrate for glass substrates (intermediate glass substrates) in thetwentieth batch were calculated, and a polishing rate ratio (thepolishing rate in the twentieth batch/the polishing rate in the firstbatch) was obtained. The number of glass substrates (glass blanks) usedin the first batch was 200. A 3.5-inch nominal size glass substrate(glass blank) having an outer circumferential diameter of 95 mm, acircular hole diameter of 25 mm, and a substrate thickness of 0.635 mmwas used as the glass substrate (the glass blank). Note that thesubstrates whose outer circumferential edge surface and innercircumferential edge surface were formed such that chamfered surfacesthereof had an angle of 135 degrees with respect to the main surfacesthereof, and had a length of 150 μm in a direction parallel to the mainsurfaces and 150 μm in the substrate thickness direction were used.

The polishing rate is a value obtained by dividing, by the polishingtime, a polishing amount obtained from the outer circumferential size ofa glass substrate (a glass blank) obtained before and after processingfor polishing the outer circumferential edge surface of the glasssubstrate. Cerium oxide loose abrasive particles were set such that D5was fixed to 0.5 μm and the particle diameter D95 was changed. At thistime, Dpeak for all cases was in a range of 1 to 5 μm. Also, D50 for allcases was in a range of 0.8 to 2.4 μm. Also, (Dpeak-D5)<(D95-Dpeak) heldtrue, and (D50-D5)<(D95-D50) held true. Also, in a particle sizedistribution where the vertical axis thereof indicates a relativefrequency, the relative frequency had two or less peaks, and in the caseof two peaks, all peaks were in a range of D10 to D85 particlediameters.

Table 1 below shows the difference between D95 and D5 and the polishingrate ratio at that time.

TABLE 1 D95-D5 [μm] Polishing rate ratio Sample 1 1 0.65 Sample 2 2 0.70Sample 3 3 0.81 Sample 4 4 0.85 Sample 5 7 0.91 Sample 6 9 0.88 Sample 711 0.87

According to the results shown in Table 1, it was found that, when D5 is1 μm or less and the difference between D95 and D5 is 3 μm or more, thepolishing rate ratio was as large as 0.80 or more, and there was no needto perform adjustment such as changing of the polishing time, forexample. Note that it was also found that it is more preferable that thedifference between D95 and D5 is 4 μm or more because the polishing rateratio further increased to 0.85 or more in such a case.

Next, loose abrasive particles whose D95 was changed in a state in whichD5 was fixed to 0.5 μm were produced, edge surface polishing wasperformed on 20 batches in a similar manner as that described above, and5 glass substrates (intermediate glass substrates) were selected fromthe twentieth batch, and, with regard to the surface quality after theabove-described edge surface polishing, surface defects of the outercircumferential edge surfaces were evaluated. Here, the specificationregarding to Dpeak, D50, the number of peaks of the relative frequencyin a particle size distribution of used cerium oxide was the same asthat of cerium oxide used in Samples 1 to 7. In the above-describedevaluations, the edge surfaces were visually observed using a digitalmicroscope, and the number of defects such as pits and scratches wasevaluated. The number of surface defects was evaluated on a scale oflevels 1 to 3. The level 1 was a level at which there were few surfacedefects, the level 2 was a level at which surface defects were slightlyobserved, and the level 3 was a level at which many surface defects wereobserved. The levels 1 and 2 are an acceptable level for practicalpurpose.

TABLE 2 Evaluation level for pits D95 [μm] in polished surface Sample 85 Level 1 Sample 9 6 Level 1 Sample 10 7 Level 1 Sample 11 8 Level 2Sample 12 9 Level 2 Sample 13 10 Level 2

According to the results shown in Table 2, it is found that D95 ispreferably 7 μm or less in the light of the fact that, even if looseabrasive particles were repeatedly used in polishing processing for along time, the quality of the polished surface can be maintained at ahigh level, and the number of defects of the polished surface wasreduced, thus increasing the surface quality.

Next, outer circumferential edge surface polishing that was the same asthat for Samples 1 to 7 shown in Table 1 was carried out using polishingliquids (Samples 14 to 18) containing cerium oxide with a particle sizedistribution in which (D95-D5) was fixed to 3 μm and D5 was changed invarious ways, and the above-described polishing rate ratios werecalculated. At this time, the specification regarding to Dpeak, D50, thenumber of peaks of the relative frequency in the cerium oxide particlesize distribution was the same as that of cerium oxide used in Samples 1to 7.

TABLE 3 D5 [μm] Polishing rate ratio Sample 14 0.3 0.80 Sample 15 0.50.81 Sample 16 0.7 0.83 Sample 17 1.0 0.82 Sample 18 1.5 0.68

According to the results shown in Table 3, it is found that, if D5exceeds 1 μm, the polishing rate ratio significantly deteriorates. It isinferred that this is because the number of particles that were very fewin the first batch and had a particle diameter of 1 μm or less rapidlyincreased in the twentieth batch.

That is, from the results shown in Table 3, it is considered importantto somewhat add abrasive particles having a particle diameter of 1 μm orless to a polishing liquid at the beginning of use with D5 being 1 μm orless, in order to suppress a change in the number of abrasive particleshaving a particle diameter of 1 μm or less.

As described above, although a polishing liquid, a method formanufacturing a glass substrate, and a method for manufacturing amagnetic disk according to the present invention have been described indetail, the present invention is not limited to the above-describedembodiment and experimental examples and it will be appreciated thatvarious improvements and modifications can be made without departingfrom the gist of the present invention.

What is claimed is:
 1. A polishing liquid that contains cerium oxide asloose abrasive particles and is to be used in polishing processing forpolishing a surface of a glass substrate, wherein, letting a particlediameter be Dx (μm) when a cumulative particle volume cumulated from thesmall particle diameter side reaches x (%) of a total particle volume ina particle size distribution obtained regarding the cerium oxide using alaser diffraction/scattering method, D5 is 1 μm or less, and adifference between D95 and D5 is 3 μm or more.
 2. The polishing liquidaccording to claim 1, wherein the difference between the D95 and the D5is 4 μm or more.
 3. The polishing liquid according to claim 1, whereinthe D95 is 7 μm or less.
 4. The polishing liquid according to claim 1,wherein the polishing liquid is used in polishing processing forpolishing an edge surface of a glass substrate with use of a brush.
 5. Amethod for manufacturing a glass substrate, comprising processing forpolishing a surface of a substrate, wherein the polishing is processingfor polishing the substrate with use of the polishing liquid accordingto claim
 1. 6. The method for manufacturing a glass substrate accordingto claim 5, further comprising processing for polishing a main surfaceof the substrate with use of a polishing liquid containing silicondioxide microparticles as loose abrasive particles.
 7. The method formanufacturing a glass substrate according to claim 5, wherein the glasssubstrate is a magnetic-disk glass substrate.
 8. The method formanufacturing a glass substrate according to claim 7, wherein themagnetic-disk glass substrate has an outer circumferential diameter of94 mm or more.
 9. A method for manufacturing a magnetic disk, comprisingprocessing for forming at least a magnetic layer on a main surface ofthe magnetic-disk glass substrate obtained using the method formanufacturing a glass substrate according to claim
 7. 10. The polishingliquid according to claim 2, wherein the D95 is 7 μm or less.
 11. Thepolishing liquid according to claim 2, wherein the polishing liquid isused in polishing processing for polishing an edge surface of a glasssubstrate with use of a brush.
 12. The polishing liquid according toclaim 3, wherein the polishing liquid is used in polishing processingfor polishing an edge surface of a glass substrate with use of a brush.13. The polishing liquid according to claim 10, wherein the polishingliquid is used in polishing processing for polishing an edge surface ofa glass substrate with use of a brush.
 14. A method for manufacturing aglass substrate, comprising processing for polishing a surface of asubstrate, wherein the polishing is processing for polishing thesubstrate with use of the polishing liquid according to claim
 2. 15. Amethod for manufacturing a glass substrate, comprising processing forpolishing a surface of a substrate, wherein the polishing is processingfor polishing the substrate with use of the polishing liquid accordingto claim
 3. 16. A method for manufacturing a glass substrate, comprisingprocessing for polishing a surface of a substrate, wherein the polishingis processing for polishing the substrate with use of the polishingliquid according to claim
 4. 17. A method for manufacturing a glasssubstrate, comprising processing for polishing a surface of a substrate,wherein the polishing is processing for polishing the substrate with useof the polishing liquid according to claim
 10. 18. A method formanufacturing a glass substrate, comprising processing for polishing asurface of a substrate, wherein the polishing is processing forpolishing the substrate with use of the polishing liquid according toclaim
 11. 19. The method for manufacturing a glass substrate accordingto claim 6, wherein the glass substrate is a magnetic-disk glasssubstrate.
 20. A method for manufacturing a magnetic disk, comprisingprocessing for forming at least a magnetic layer on a main surface ofthe magnetic-disk glass substrate obtained using the method formanufacturing a glass substrate according to claim 8.